Global demand for diesel has risen quickly with increased growth of transportation fuels. At the same time, regulations on the properties of the transportation diesel have become more rigorous in order to mitigate environmental impact. European standards, for example, call for a density less than 860 kilograms per cubic meter (kg/m3), a polycyclic aromatics content of less than 11 wt. % and a sulfur content of less than 10 part per million by weight (wppm) which is often referred to as ultra-low-sulfur-diesel, or ULSD. Future standards call for a density less than 845 kg/m3.
There is a need for a broader range of hydrocarbon feeds to use as feedstocks for producing diesel, including ULSD. A refinery produces a number of hydrocarbon products having different uses and different values. It is desired to reduce production of or upgrade lower value products to higher value products. Lower value products include cycle oils which have historically been used as blend-stock for fuel oil. However, such oils cannot be directly blended into today's diesel fuels because of their high sulfur content, high nitrogen content, high aromatics content (particularly high polyaromatics), high density, and low cetane value.
Various hydroprocessing methods, such as hydrodesulfurization and hydrodenitrogenation, can be used to remove sulfur and nitrogen from a hydrocarbon feed. Additionally, hydrocracking, can be used to crack heavy hydrocarbons (high density) into lighter products (lower density) with hydrogen addition. However, high nitrogen content can poison a zeolitic hydrocracking catalyst, and hydrocracking conditions which are too severe can cause the formation of significant amounts of naphtha and lighter hydrocarbons which are considered lower value products.
Thakkar et al. in “LCO Upgrading A Novel Approach for Greater Value and Improved Returns” AM, 05-53, NPRA, (2005), propose a once-through hydrotreating and hydrocracking flow scheme for upgrading a light cycle oil (LCO) into a mixture of liquefied petroleum gas (LPG), gasoline and diesel products. Thakkar et al. disclose producing a low sulfur content diesel (ULSD) product. However, Thakkar et al. use traditional trickle bed reactors. Significant amounts of light gas and naphtha are produced in the disclosed hydrocracking process. The diesel product accounts for only about 50%, or less, of the total liquid product using LCO feed.
Leonard et al. in U.S. Pat. No. 7,794,585 disclose a process for hydrotreating and hydrocracking hydrocarbon feedstocks in a “substantially liquid phase” which is defined as the feed stream has a larger liquid phase than a gas phase. More specifically, hydrogen may be present in a gas phase up to 1000 percent of saturation. Leonard et al. teach such high amounts are needed so that as hydrogen is consumed, hydrogen is available from the gas phase. Thus, the Leonard et al. reaction system is a trickle bed.
Conventional three-phase (trickle bed) hydroprocessing units used for hydrotreating and high pressure hydrocracking require hydrogen from a vapor phase to be transferred into liquid phase where it is available to react with a hydrocarbon feed at the surface of the catalyst. These units are expensive, require large quantities of hydrogen, much of which must be recycled through expensive hydrogen compressors, and result in significant coke formation on the catalyst surface and catalyst deactivation.
U.S. Pat. No. 6,123,835, discloses a two-phase (“liquid-full”) hydroprocessing system which avoids some the disadvantages of trickle bed systems.
U.S. Patent Application Publication 2012/0205285 discloses a two-stage process for targeted pretreatment and selective ring-opening in liquid-full reactors with a single recycle loop to convert heavy hydrocarbons and light cycle oils to liquid product having over 50% in the diesel boiling range.
Still, it is desirable to provide hydroprocessing systems which convert heavy hydrocarbon feeds, in particular LCO, to diesel in higher yield and/or quality.